The present invention generally relates to power system control, and more specifically, to an apparatus and method for controlling operation of a single-phase voltage regulator in a three-phase power system.
Electric utility systems or power systems are designed to generate, transmit and distribute electrical energy to loads. In order to accomplish this, power systems generally include a variety of power system elements such as electrical generators, electrical motors, power transformers, power transmission lines, distribution lines, buses and capacitors, to name a few. As a result, power systems must also include a number of regulators having associated control devices, and many protective devices having associated protective schemes to protect the power system elements from abnormal conditions such as electrical short circuits, overloads, frequency-excursions, voltage fluctuations, and the like.
In general, protective devices and their associated protective schemes act to isolate a power system element(s) (e.g., a generator, transformers, buses, motors, etc.) from the remainder of the power system upon detection of the abnormal condition or a fault in, or related to, the power system element(s). Such protective devices may include different types of protective relays, surge protectors, arc gaps and associated circuit breakers and reclosures.
Regulators and their associated control devices are utilized to regulate the voltage level in the power system. For example, a number of single-phase step voltage regulators may be coupled to the various transmission, sub-transmission and distribution lines (collectively, “distribution lines”) to enable voltage regulation of the distribution line to, for example 13 kV±10 percent, during a wide range of load conditions (e.g., a plant coming on-line). Such voltage regulators are often placed adjacent to a step-down power transformer and generally include an autotransformer having a single winding (e.g., a series winding), which is tapped at some tap position along the winding to provide a desired voltage flow.
A typical step voltage regulator may have a 100 percent exciting winding in shunt with the distribution line on the source side, and operate to maintain a voltage on the load side of the distribution line. The voltage is maintained within a desired voltage bandwidth by means of a 10 percent tapped buck/boost winding connected in series with the distribution line. The series winding has taps connected to stationary contacts of a tap changer dial switch, where the tap changer dial switch includes a pair of rotatable selector contacts driven by a reversible motor into sequential engagement with the pairs of contacts. For example, the tap changer dial switch may enable an ability to change the effective turns ratio from input to output±10 percent in 32 steps of 5/8 percent each or 0.7 V. A voltage control device monitors the distribution line voltage and current and determines the proper tap position based on the measured distribution line voltage.
Voltage regulators operate via a comparison of an actual measured voltage (i.e., a secondary distribution line voltage) to some internal fixed reference voltage, or center-band voltage. Any voltage difference is amplified and used to control operation of the voltage regulator via the voltage control device. Thus, if the measured voltage is too high or in an out-of-band (OOB) area above a center-band area or center-band voltage range, the voltage regulator is commanded by the voltage control device to execute a tap position change to produce a lower voltage, and if the voltage is too low, or in an OOB area below the center-band area, the voltage regulator is commanded by the voltage control device to execute a tap position change to produce a higher voltage.
Because currents resulting from a fault can easily exceed 10,000 amperes (amps) and because the voltage control device is designed to utilize currents and voltages much less than those of the distribution lines, the currents and voltages are stepped-down via current and voltage transformers, respectively. As is known, the three-phase current and voltages are commonly referred to as the primary current and voltages, while the stepped-down current and voltages are referred to as the secondary current and voltages, respectively. The stepped-down secondary current and voltages are digitized and then utilized by a microcontroller of the voltage control device to determine corresponding phasors representative of the primary current and voltages. The phasors are then used by the microcontroller while executing the voltage control logic scheme of the voltage control device to determine whether a tap position change is required by the voltage regulator (discussed below).
One voltage control scheme commonly referred to as a definite time characteristic, includes setting a countdown timer, referred to herein as a First timer, upon detection of a measured voltage in an OOB area. Such a voltage excursion into the OOB area is determined by comparing a voltage phasor, calculated from secondary voltages provided by the voltage transformer, to the center-band area. If the measured voltage remains in the OOB area during a countdown time period of the First timer, a tap position change is initiated to either lower the load voltage (due to a high OOB voltage) or raise the load voltage (due to a low OOB voltage). If the measured voltage does not remain in the OOB area for the countdown time period, and instead the measured voltage dips in-band momentarily or otherwise, the First timer resets to its countdown time period. The First timer will again begin its countdown time period upon detection of a second voltage excursion into the OOB area. As a result, the elapsed time period of the first voltage excursion into the OOB area is ignored. If the voltage again dips in-band, the First timer, executing the countdown for second time, again resets to it countdown time period. Thus, for cases where the measured voltage is oscillating around the in-band/OOB edge (i.e., dipping in and out of the in-band area), the voltage control device may not issue a needed tap position change command to the voltage regulator due to repeated First timer resets. Accordingly, the feedline voltage is not optimized to the in-band area.